1. Field
The following description relates to an image forming apparatus and a method for color registration adjustment and, more particularly, to an image forming apparatus which detects an area with an undesirable surface condition of an image forming medium and a method for correction color registration.
2. Description of the Related Art
An electrophotographic printer, such as a color laser printer, for example, generally includes four photosensitive drums Dy, Dc, Dm, and Dk corresponding to four colors, yellow, cyan, magenta, and black, a light exposing device which irradiates light onto the respective photosensitive drums Dy, Dc, Dm, and Dk to form an electrostatic latent image of an intended image, a developing device which develops the electrostatic latent image with developers of respective colors, and an image forming medium. The image forming medium (or transfer belt or intermediate transfer belt) receives developer images formed on the respective photosensitive drums Dy, Dc, Dm, and Dk in sequence and in an overlapping manner, forms a complete color image, and transfers the color image onto a printing paper.
Accordingly, to print out one color image, an image is developed on the four photosensitive drums Dy, Dc, Dm, and Dk in respective colors, and the developer images are overlappingly printed at a same location on the transfer belt to thus form a final color image. The final color image is then printed on a printing paper.
Meanwhile, in order to form an intended color image in an intended form precisely at the same intended location on the transfer belt, it is very important that the developer images in four colors transferred from the photosensitive drums Dy, Dc, Dm, and Dk start on the transfer belt exactly at the same location and also end exactly at the same location. Even if the images on the four photosensitive drums Dy, Dc, Dm, and Dk are developed in crisp and clear form, if any developer images are transferred in even a slightly misaligned arrangement, it is not possible to obtain a final color image with clear color and form based on such developer images.
Accordingly, in order to form an intended color image in precisely-intended color and form, it is important to align the location at which the exposure device starts exposing light onto the respective photosensitive drums Dy, Dc, Dm, and Dk by considering the moving speed of the transfer belt, so that the plurality of developer images overlap each other at a precisely-aligned location. This alignment of light exposure location is ‘color registration’.
The color registration is generally carried out using a registration sensor equipped in the image forming apparatus. The registration sensor irradiates light onto a registration pattern developed on the image forming medium, and measures error in the image alignment by detecting the reflected light.
In order to accurately measure the error in image alignment, the registration sensor irradiates light onto the image forming medium based on an accurate luminance value. To be specific, if the luminance of the registration sensor is higher than a preset value, part of the registration pattern developed on the transfer belt may not be perceived. If the luminance of the registration sensor is lower than the preset value, contaminants within the transfer belt may be perceived as part of the pattern.
It is also necessary that the registration pattern is formed on a non-contaminated area on the image forming medium to ensure that the error in image alignment is measured accurately.
In this respect, conventionally, the operation of finding luminance of the registration sensor is carried out first, which is then followed by the operation of rotating the image forming medium one time to find a contaminated area which may exist on the image forming medium.
However, since the operation to find the luminance of the registration sensor is also carried out by rotating the image forming medium one time, the image forming medium is rotated once each time to find the luminance of the registration sensor and also to find the contaminated area on the image forming medium, and it takes some time until the two operations are finally completed.